Among the prior art surge voltage devices those consisting of a gas-filled tube having two electrodes may be mentioned, the tube being connected with one electrode to the line to be protected, and with the other electrode to ground. In operation such surge voltage protectors resemble glow-lamps. Their voltage limitation occurs only above a voltage of 200 volts. Therefore, voltage limiting devices of this nature cannot be applied for the protection of circuits including semiconductor devices, because surge voltages of that magnitude would result in a breakdown of the semiconductor.
Furthermore, so-called varistors have been applied as surge voltage protectors. Varistors consist of a semiconductor body with two electrodes. The resistance between the electrodes is independent of the direction of current flow and dependent upon the applied voltage. This is one of the limitations of the application of varistors for surge voltage protection. Varistors have the disadvantage of forming non-negligible shunts across the circuitry to be protected, even if the applied voltage is small. This is another limitation of the application of varistors for surge voltage protection.
For conducting surge voltages to ground diodes or series arrangements of diodes and/or Zener diodes have been proposed. These are, however, if available space is limited only suitable for dissipating small power surges. Zener diodes have the further disadvantage, particularly if monolithically integrated, of having substantial individual deviations.
It is, therefore, the principal object of this invention, to provide surge voltage protective devices not subject to the limitation of the above prior art devices.
A more special object of this invention is to provide means which limit a predetermined voltage approximately to a magnitude in the order of 3 volts, without significant deviations from the required value, and which are not bulky and capable of carrying currents in the order of 1 amp. without substantial voltage drop.